1. Field of the Invention
The present invention generally relates to transmitting signals between discrete integrated circuit devices and more specifically to single-ended signaling techniques.
2. Description of the Related Art
Single-ended signaling systems employ a single signal conductor per bit stream to be transmitted from one integrated circuit device (chip) to another chip. By way of contrast, differential signaling systems explicitly require two signal conductors, so single-ended signaling is often assumed to be an advantage in cases where the number of off-chip pins and signal conductors is limited by packaging constraints.
However, single-ended signaling systems actually require more circuitry per channel than just one signal conductor. The current that flows from transmitter to receiver must be returned to the transmitter to form a complete electrical circuit, and in single-ended signaling systems the current that is returned flows over a set of shared conductors, typically the power supply terminals. In order to keep the return current flow physically close to the signal conductor, the shared return terminals are usually physical planes in the packaging, e.g., chip package or printed circuit board, allowing the signal conductors to be constructed as strip-lines or micro-strips. Therefore, single-ended signaling systems always require>N pins and conductors to carry N bit streams between chips, and this overhead is typically on the order of 10-50%.
Single-ended signaling systems require a reference voltage at the receiver in order for the receiver to distinguish between (typically) the two signal levels that represent a “0” and “1”. By contrast, differential signaling systems do not require a reference voltage: the receiver need only compare the voltages on the two symmetric conductors of the differential signaling system to distinguish the data value. There are many ways to establish a reference voltage for a single-ended signaling system. However, it is fundamentally difficult to ensure agreement on the value of the reference voltage between transmitter and receiver and agreement is needed to ensure consistent interpretation of the signals sent by the transmitter to the receiver.
Single-ended signaling systems dissipate more power for a given signal-to-noise ratio compared with equivalent differential signaling systems. In the case of resistively terminated transmission lines, a single-ended system must drive a current of +V/R0 to establish a voltage V above the reference voltage at the receiver for a transmitted “1”, and sink current −V/R0 to establish a voltage of V below the reference voltage at the receiver for a “0”, where R0 is the termination resistance. Thus the system consumes a current of 2V/R0 to establish the required signal at the receiver. In comparison, when differential signaling is used, the transmitter only need drive current of ±V/2R0 to establish the same voltage (V) across the receiver terminals, thanks to the symmetric pair of signal conductors. A differential signaling system only needs to draw V/R0 of current from the power supply. Therefore, even assuming a reference voltage at the receiver that is perfectly matched to the transmitter, single-ended signaling systems are fundamentally half as power-efficient as differential ones.
Finally, single-ended systems are more susceptible to externally coupled noise sources compared with differential systems. For example, if noise is electromagnetically coupled to a signal conductor of a single-ended system, the voltage that arises from this coupling arrives as un-cancelled noise at the receiver. The noise budget for the signaling system must therefore account for all such noise sources. Unfortunately, such noise coupling is often from neighboring wires in a bundle of single-ended signals, called cross-talk, and this noise source is proportional to the signal voltage level, and therefore cannot be overcome by increasing the signal level. In differential signaling, the two symmetric signal conductors can be run physically close to one another between a transmitter and receiver so that noise is coupled symmetrically into both conductors. Thus many external noise sources affect both lines approximately identically, and this common-mode noise can be rejected at a receiver that has higher differential gain than common-mode gain.
Accordingly, what is needed in the art is a technique for providing single-ended signaling while reducing the problems of establishing a reference voltage, reducing the shared impedance of the signal return path and crosstalk caused by the signal return path, and reducing the power consumption of the single-ended signaling system.
FIG. 1A shows an example prior art single-ended signaling system 100, sometimes called a “pseudo-open-drain” (PODL) system that illustrates the reference voltage problem. Single-ended signaling system 100 includes a transmitting device 101 and a receiving device 102. The transmitting device 101 operates by drawing current Is from the power supply when sending a “0”, and drawing no current when sending a “1” (allowing the terminating resistors R0 and R1 to pull the signal up to Vdd). To develop a signal of magnitude |V| at the receiving device 102, the signal swing must be 2V, so current Is=2V/(R0/2) when driving a “0”, and 0 otherwise. Averaged over an equal number of “1's” and “0's”, the system consumes 2V/R0 from the power supply.
The signal at the input of the receiving device 102 swings from Vdd (“1”) down to Vdd−2V (“0”). To distinguish the received data, the receiving device 102 needs a reference voltage of Vref=Vdd−V. There are three ways to generate the reference voltage as shown in FIGS. 1A, 1B, and 1C.
As shown in FIG. 1A, an external reference voltage Vref is generated by a resistor network located close to the receiving device. The external reference voltage passes into the receiving device through a dedicated pin 103 and is distributed to some number of receivers that share the external reference voltage. A first problem with the external reference voltage technique shown in FIG. 1A, is that the external reference voltage is developed across external resistors R2a and R2b between the power supply terminals Vdd and GND, and that the generated external reference voltage cannot be matched with the voltage developed by the current sources in the transmitting device 101, since the current sources are completely uncorrelated with the external resistors R2a and R2b. 
A second problem is that the power supply voltage at the receiving device 102 may be different from the power supply voltage at the transmitting device 101, since the supply networks to the two communicating chips have different impedances, and the two chips draw different, and variable, currents. A third problem is that noise injected into any one of the signaling wires 105 coupling the transmitting device 101 to the receiving device 102 is not injected into the reference voltage, and therefore the signaling system must budget for the worst case noise voltage that may be introduced to the signaling wires 105. A fourth problem is that the voltage level between the external power supply terminals Vdd and GND differs from the internal power supply network within the receiving device 102, again because of the supply impedance. Furthermore, the configuration of the single-ended signaling system 100 causes the currents in the shared supply terminals to be data dependent. Thus, any data dependent noise that is introduced to the inputs of the internal receiver amplifiers within the receiving device 102 differs from the external supply noise that is introduced to the shared external reference voltage that is also input to the internal receiver amplifiers.
FIG. 1B shows an example prior art single-ended signaling system 120, that uses an internal reference voltage. The internal reference voltage Vref attempts to improve on the noise problems compared with the single-ended signaling system 100 that uses an external reference voltage. The single-ended signaling system 120 also tracks the reference voltage level of the transmitting device 121 more closely compared with the single-ended signaling system 100. A scaled transmitter is included in the receiver circuitry of the receiving device 122 that generates an internal Vref that is related to the termination resistance and the transmitter current Is. Because the internal reference voltage is generated relative to the internal power supply network, the internal reference voltage does not suffer the supply-noise problems of the external voltage reference shown in FIG. 1A. However, the noise coupling problems of the external reference voltage approach described in conjunction with FIG. 1A remain. Further, because the current source (Is/2) used to generate the internal reference voltage is in a different chip than the current sources in the transmitting device 121, the current source in the receiving device 122 may not track the current sources in the transmitting device 121.
FIG. 1C shows an example prior art single-ended signaling system 130, that uses a bundled reference voltage Vref. Tracking between the bundled reference voltage and signal voltages is improved because the bundled reference voltage is generated in the transmitting device 131 using a scaled transmitter and the bundled reference voltage is coupled to the same internal supply network as the data transmitters in the transmitting device 131. Therefore, the bundled reference voltage can be made to track the transmitter device's 131 process-voltage-temperature variations reasonably well. The bundled reference voltage is transmitted from the transmitting device 131 to the receiving device 132 over a wire that is parallel to and as identical as possible to the signaling wires 135 that transmit the data.
External noise that may be coupled into the system, including some components of power supply noise, can be cancelled since the external noise appears as common-mode noise between the bundled reference voltage and any given signal of signaling wires 135. Cancellation of the common-mode noise cannot be perfectly effective however, because the bundled reference voltage has terminating impedance at the receiving device 132 that is different from a data signal; since the bundled reference voltage must be fanned out to a large number of receivers, the capacitance on the pin receiving Vref is always larger than on a typical signal pin, so noise is low-passed, relative to a data signal.
FIG. 2A shows the current flow in a prior art single-ended signaling system 200 in which the ground plane is intended to be the shared signal return conductor. Single-ended signaling system 200 illustrates the previously described return impedance problem. As shown in FIG. 2A, the single-ended signaling system 200 is transmitting a “0” by sinking current at the transmitting device 201. Half of the current flows out over the signal conductor (the signal current flow 204), and the other half, the transmitter current flow 203, flows through the terminator of the transmitting device 201. In this example, the return current is intended to flow in the ground (GND) plane, and if the signal conductors are referred to the ground plane, and no other supply, electromagnetic coupling between signal and ground plane will cause image currents to flow in the ground plane immediately below the signal conductor. In order to achieve a 50/50 current split at the transmitter, a path for the transmitter's local current, transmitter current flow 203 through the terminating resistor 206 to return to the ground is provided by an internal bypass capacitor 205 in the transmitting device 201. The signal current 204 is returned to the receiving device 202 through the ground plane and is intended to flow into receiving device 202 through its bypass capacitor 207 and internal terminating resistor 208.
FIG. 2B shows the current flow in the prior art single-ended signaling system 200 in which the ground plane is intended to be the shared signal return conductor when a “1” is transmitted from the transmitting device 201 to the receiving device 202. The current source in the transmitting device 201 is off (and is not shown), and the terminating resistor 206 within the transmitting device 201 pulls the line HI. Again, the return current is intended to flow in the ground plane, so the bypass capacitor 215 in the transmitting device 201 is required to carry the signal current flow 214. The current path in the receiving device 202 is the same as for transmitting a “0”, as shown in FIG. 2A, apart from direction of transmitted current flow.
There are several problems with scenario shown in FIGS. 2A and 2B. First, if the impedance of the bypass capacitors 205 and 215 is not low enough, some of the signal current will flow in the Vdd network. Any redirected current flowing through the Vdd network will somehow have to rejoin the image current in the ground network, and to reach the ground plane the redirected current will have to flow through external bypass capacitors and other power supply shunt impedances.
Second, the return current flows through the impedance of the shared ground network 211 at the transmitting device 201 and the shared ground impedance 212 at the receiving device 202. Since ground is a shared return path, the signal current produces a voltage across the ground network impedances 211 and 212. At the receiving device 202, the voltage across the ground network impedances 211 and 212 produces noise in neighboring signal paths, providing a direct source of crosstalk.
Third, if the shared ground return pin is some distance from the signal pin for which the ground pin provides a return path, there is an inductance associated with the current loop that is formed, increasing the effective ground impedance, and exacerbating the cross-talk between signals that share the ground pin. In addition, the inductance is in series with the terminator and will cause reflections in the signaling channels, yet another source of noise.
Finally, the current flow shown in FIGS. 2A and 2B is the transient flow that occurs when a data edge is transmitted. The steady-state flow is quite different, in both cases, since the current must flow through the power supply over both the Vdd and ground networks. Since the steady-state current path is different from the transient one, there is a transition between the two conditions in which transient current flows in both Vdd and ground networks, dropping voltages across the supply impedances, and generating more noise.
One may assume that it is preferable to use the Vdd network to carry the return currents, since the termination resistances are connected to this network. This choice would not solve the basic problem, however. The bypass capacitors 205 and 215 are still needed to route transient signal currents, and the transient and steady-state conditions still differ, so there is still cross-talk from shared supply impedances, and voltage noise from data transitions. The fundamental problems are two-fold: first, the shared supply impedances are a source of cross-talk and supply noise. Second, the split in signal current between the two supplies makes it difficult to keep the return current physically adjacent to the signal current through the channel, which leads to poor termination and reflections.
In the single-ended signaling system 200, the current drawn from the power supply is data dependent. When transmitting a “0”, the transmitter sinks current Is. Half of the current flows from the power supply into the receiving device 202, through the terminator, back over the signal wire, then through the current source in the transmitting device 201 to ground, and thence back to the power supply. The other half of the current flows from the power supply into the Vdd network of the transmitting device 201, through the terminator of the transmitting device 201, then through the current source, and back through the ground network.
When transmitting a “1”, the steady state is one in which no current flows through the power supply network at all. Therefore, when data is toggling between “1” and “0”, the peak-to-peak current in the Vdd and ground network of the transmitting device 201 is 2×the signaling current, and lx in the receiving device 202; the varying current creates voltage noise on the internal power supplies of each of the transmitting device 201 and the receiving device 202 by dropping across the supply impedances. When all of the data pins that share a common set of Vdd/ground terminals are switching, the noise in the shared impedances is additive, and the magnitude of the noise comes directly out of the signaling noise budget. It is difficult and expensive to combat this noise: reducing the supply impedances generally requires providing more power and ground pins and/or adding more metal resources on chip to reduce impedances. Improving on-chip bypass costs in terms of area, e.g., for large thin-oxide capacitors.
A solution to address all three of the reference voltage, the return impedance, and the power supply noise problems is to employ differential signaling. The reference problem is non-existent when differential signaling is used. The return impedance problem is gone, thanks to the symmetric second signaling wire, which carries all of the return current. The power supply current is nearly constant and independent of the data being transmitted. However differential signaling requires twice as many signal pins as single-ended signaling, as well as the overhead of some number of power/ground pins.
Accordingly, what is needed in the art is a technique for providing single-ended signaling while reducing the problems of establishing a reference voltage, reducing the impedance of the signal return path, and reducing the power supply noise.